1. Field of the Invention
The present invention relates to a high-frequency component for use in high-frequency circuits of mobile communications equipment, e.g., portable telephones, for distributing or coupling two high-frequency signals in different frequency ranges.
2. Description of the Related Art
FIG. 9 illustrates a duplexer 50 as one example of a conventional high-frequency component for distributing or coupling high-frequency signals. The duplexer 50 comprises a low pass filter (LPF) 51 and traps 52, 53 each constructed as a parallel resonator. The LPF 51 and the trap 52 cooperatively make up a low-frequency filter. Thus, a signal input to a first port 54 is output from a second port 55 through the trap 52. On the other hand, the trap 53 constitutes a high-frequency filter. Thus, a signal input to a third port 56 is output from the second port 55 through the trap 53.
Because of using discrete components such as the LPF 51 and the traps 52, 53, the duplexer 50 constructed as explained above has however a problem that it has a large size and cannot easily meet current demands in the field of mobile communications equipment, for example, where the device size will presumably be further reduced in the future.
With a view toward solving the above problem, Japanese Unexamined Patent Publication No. 6-85506 discloses a duplexer 60 as shown in FIG. 10. The duplexer 60 is constructed such that a high-frequency filter 61 and a low-frequency filter 62 each comprise a plurality of dielectric layers (not shown in detail) which are stacked and laminated one above another and provided with inductance forming electrodes and capacitance forming electrodes. The filters 61 and 62 are built in a single multilayer substrate 64 with a shielding electrode 63 stacked and laminated in parallel between the respective sets of layers that form the filters 61 and 62.
In the high-frequency component (duplexer) shown in FIG. 10, the shielding electrode must be provided between the high-frequency filter and the low-frequency filter to shield the high-frequency filter and the low-frequency filter from each other for suppressing mutual interference. Since the respective layers and the shielding electrode 63 are all stacked in parallel, it is relatively easy to insert the shielding electrode 63 between the filters 61 and 62 while the multilayer substrate 64 is being manufactured.
On the other hand, it is desirable for the high-frequency filter and the low-frequency filter to be formed side by side horizontally in order to reduce the height of the duplexer. In this case (not shown), a difficulty is encountered in forming the shielding electrode, if each filter is formed by laminating the respective plurality of dielectric layers one above another in the direction of height. Since the shield electrode must be disposed between the first filter and the second filter, the shield electrode must be disposed perpendicularly to the layers that make up the filters. The shield electrode must be accurately inserted between the two filters after they are manufactured, which is very difficult. This problem places a limitation upon the use of a duplexer in which the high-frequency and low-frequency filters are disposed side-by-side.